The GP2x now has a nice Atari 2600 emulator. It’s an port of Stella by Alex Zaballa. Release notes:

Rough GP2X Implementation of the popular Stella emulator. Sound is not working but you can enjoy save state capability and smooth gameplay. I plan to fix the sound and the loader as soon as possible. But to give you an idea of what I’ve been doing here is my first release of Stella for the GP2X. Enjoy.